Particle counters and other instruments for detecting particles that are small compared to the wavelength of light, are typically obliged to detect particles at the focus of an optical beam. This enables a resulting signal to be enhanced by focussing the optical beam to a very small intense region. In such systems, particles traverse the focus region sufficiently quickly, even at low flow rates, that particle transit times are short compared to the time constants of mechanical vibrations and other noise sources. Further, the detection volume can be isolated to a region of the particle-containing fluid that is far from the windows between the optical elements and the fluid.
If the optical beam is non-uniform throughout the sensing volume, it is inevitable that particles of the same size, travelling with the fluid in different parts of the sensing volume, will produce different signals. The simplest cure for that effect is to restrict the fluid flow through an orifice that is sufficiently small that the optical response to a particle is essentially constant throughout the entire volume. This is impractical in strongly focussed systems because an orifice that is small will inevitably clog; large pressure drops are associated with fluids passing through such small orifices; and orifice walls interfere with the optical path.
The prior art illustrates many instruments and methods for detecting particles by measurement of "scattered" light intensity from a particle or collection of particles. "Forward direction" scattered light is generally excluded from the such measurements due to the presence of the incident beam. Forward direction scattered light is also known as "bright field" light. When the bright field is excluded (e.g. by masking), a "dark field" scattered light pattern results. It is known, however, that the relationship between a forward direction scattered light field from a small particle and a focussed incident beam is such that the particle causes a phase shift and an attenuation of the incident beam. The attenuation is called the extinction effect.
In U.S. patent application Ser. No. 07/184,639, now U.S. Pat. No. 5,061,070, entitled "Particulate Inspection of Fluids Using Interferometric Light Measurements", by Batchelder et al., and assigned to the same assignee as this application, the phase shift experienced by the forward direction field of an incident beam is employed to differentiate between bubbles and particles in a fluid. Batchelder et al. show that a small dielectric particle in a focussed, monochromatic light beam, produces a scattered wave in phase quadrature with the far-field incident beam. The forward direction scattered light is detected using an interferometer which measures the phase shift of one beam relative to another. A similar teaching by Batchelder et al. appears in Applied Physics Letters, Vol. 55, No. 3, July 1989, pp. 215-217.
In U.S. patent application, Ser. No. 07/547,735, now U.S. Pat. No. 5,037,202, entitled "Measurement of Size and Refractive Index of Particles Using the Complex Forward-Scattered Electromagnetic Field", by Batchelder et al., and assigned to the same assignee as this application, the phase shift and extinction experienced by a pair of incident beams are employed to characterize particles in a fluid. The two beams are orthogonally polarized and changes in signals derived from those beams (that occur when a particle passes therethrough) enable the particle to be classified with respect to its refractive index, and thus identified.
In many particle measuring systems which solely employ dark field scattered light (as contrasted to the forward-direction field employed by the above two referenced patent applications), signals derived from small particles therein show a dependence on the particle's trajectory. If the trajectory of the particle moves from a beam's focal plane, the signal waveforms are modified
These modifications create non-uniform waveform shapes which are difficult to analyze.
One way to compensate for such non-uniform waveforms is described by Hirleman in "Laser Technique for Simultaneous Particle-Size and Velocity Measurements", Optics Letters, Vol. 3, No. pp. 19-21, (1978) and in U.S. Pat. No. 4,251,733. Hirleman describes how detected light-scatter exhibits several peaks in time as a particle travels through a two beam optical system. The trajectory of the particle is deduced from the height of those two peaks (see FIG. 4 of the patent and FIG. 3 of the Optics Letters article).
A somewhat more complicated trajectory correction system is described by Knollenburg in U.S. Pat. No. 4,636,075. Knollenburg employs coaxial beams of orthogonal polarization and coincident focal planes but differing spot sizes. Knollenberg's sensor requires that the signal from the tightly focussed spot rise above a threshold before taking the signal from the larger spot to obtain particle size.
In U.S. Pat. No. 4,662,749 to Hatton et al., a fiber optic probe is employed to project a fringe image into a measurement zone and then sense the resulting light perturbations which occur when a particle traverses the zone. Those perturbations are employed to enable calculation of the velocity and/or size of the particles.
A number of patents illustrate the use of several light beams to determine particle trajectory and other parameters such as velocity; however, in general, they employ dark field methods which do not allow maximal use of the available light intensity In U.S. Pat. No. 4,387,993 to Adrian, a dark field method is described in which concentric, annular incident beams of different focal spot size are used to determine whether a particle's trajectory is in the center of the larger beam. In U.S. Pat. No. 4,537,507 to Hess, a dark field method is described in which crossed beams of different focal spot size form a detection region with varying intensity patterns. In U.S. Pat. No. 4,540,283 to Bachalo, a dark field system is described in which two incident beams are crossed to set up an interference pattern that creates a varying signal when a particle passes therethrough. U.S. Pat. No. 4,764,013 to Johnston describes a dark field method that determines the phase difference between two polarization components of scattered light.
U.S. Pat. No. 4,788,443 to Furuya describes a dark field system wherein a single beam is projected into a particle-containing fluid. The scattered light is converted into electrical signals that are counted to derive the diameter and/or concentration of the particles. U.S. Pat. No. 4,854,705 to Bachalo describes another dark field system wherein concentric beams of different focal spot size are used to determine a particle's trajectory, in conjunction with detection optics which are confocal, thereby limiting the detection volume further. Carr et al. in U.S. Pat. No. 4,927,268 also describe a dark field method in which focal spots of varying size are used to determine whether a particle passes through the center portion of a larger beam. Optical fibers are employed to reduce alignment problems.
Accordingly, it is an object of this invention to provide an optical system which employs a bright field detection system for both detecting particles and determining their trajectories.
It is another object of this invention to provide an optical system for detecting particles in a fluid, which system compensates for particle trajectories that are not in the focal plane of an optical beam.
It is a further object of this invention to provide an optical system for measurement of extinction arising from beam/particle interaction, wherein the particle's flow direction is both known and unknown.
It is yet another object of this invention to provide a system for measurement of extinction arising from beam/particle interaction wherein artifacts in the light path are negated.